United have had great support but it has no doubt been helped by the success they had in the 50's and 60's and then again in the 90's onwards. Also, the successful northern teams tend to pick up a lot of the Irish fan base as well (see Liverpool and United in particular).
Nothing wrong with this as such, as all clubs that have sustained success will pick up glory hunters along the way. I just can't take southern United/Liverpool fans that seriously to be honest. We could all go and support the best but it does feel sweeter to have had the hard times (80's!!) and gradually got better to the point we are at now. I'd feel the same about a northern Chelsea fan generally.